This invention relates to a drill blank and more particularly a micro-drill blank.
In the manufacture of various products, particularly printed circuit boards, it is necessary to form a plurality of small holes therein. These holes are generally formed by means of a small diameter twist drill or micro-drill. Since such boards are abrasive, for example made of fibre reinforced plastic, it has been found that drill bits tipped with diamond abrasive compact are particularly suitable.
Various methods have been described in the literature for producing micro drills. EP 0 206 652 describes a method of producing a blank for a drill bit comprising providing a composite abrasive compact having a diamond or cubic boron nitride compact bonded to a cemented carbide backing and having major surfaces on each of opposite sides thereof, severing the composite abrasive compact along planes perpendicular to the flat surfaces and from one flat surface to the other to produce a plurality of sticks polygonally-shaped end-on, each severed plane defining side surfaces of adjacent sticks and longitudinally rounding each stick to produce from it a blank for a drill bit which is round end-on. The drills may be made by fluting the rounded stick.
EP 0 223 474 describes a method of making a drill blank comprising a cylindrical carbide body having a vein of abrasive compact embedded in a groove at one end thereof and bonded to the carbide. The method includes the steps of providing a solid, cylindrical body of cemented carbide having end surfaces joined by a side surface, forming a series of grooves in one end surface, placing compact-forming material in the grooves, exposing the cylindrical body to temperature and pressure conditions suitable to produce an abrasive compact of the material in the grooves and severing the cylindrical body longitudinally from one end surface to the opposite end surface to produce a plurality of drill blanks. The drills are again produced from such blanks by fluting.
The drill blanks described above all utilise abrasive compact at the working end thereof. Abrasive compact, which may be diamond or cubic boron nitride abrasive compact, is extremely hard and abrasion resistant. Consequently, if, as happens in fluting the blank, much abrasive compact has to be removed, the cost of the drill bit itself increases. In both of the methods described above, the abrasive compact extends from one side surface of the blank to the other creating a situation in which quite a bit of abrasive compact has to be removed in fluting the blank.
Other methods of making micro drill bits are known. Some of these methods, e.g. that described in EP 0 132 652, involve various bonding steps which again adds to the cost. The bonded regions also introduce potential zones of weakness.